A directional control valve, as the name connotes, controls the flow path patterns in a hydraulic or pneumatic system. Interposed between ports in the system, it directs the fluid flow in various ways between these ports, or stops the flow at some of the ports, in a predetermined pattern according to its position. Most directional control valves have either two or three working positions, each determining a different flow pattern; generally, the more ports a valve serves, the more positions it will have. The position of a valve is determined by one or more operators, also called actuators. Operators may provide for direct manual control, may follow other components, as by cams, or may be remotely controlled, for example, with solenoids.
A great variety of directional control valve designs exists. One common type of design is the sliding-action spool-type valve. This valve has a cylindrical spool which slides axially in a cylindrical bore with which the ports communicate. The spool has raised sections, called lands, which closely fit the cylindrical bore. Between the lands are sections where the spool diameter is substantially less than that of the bore, so that annular chambers are created. Axial flow across a land on the outside of the spool is prevented, but if two ports both open into the bore at points not separated by a land, they are permitted to communicate via the annular chamber between the spool and the bore. A change in axial position of the spool will dispose the lands and annular chambers differently with respect to the ports and thus change the flow pattern.
In a transmission, several types of directional flow functions are ordinarily required, each calling for a different spool configuration, for example,
(1) Forward - Neutral - Reverse, calling for a 3-position, 4-way (4 port) valve; PA1 (2) High Clutch - Low Clutch, calling for a 2-position, 4-way valve; and PA1 (3) Torque Converter Lockup, calling for a 2-position, 4-way valve, functioning as a 3-way valve in one position.